ABSTRACT Microsporidia are remarkable parasites related to the Fungi that have been studied for more than 150 years. They are remarkable in their exploitation of all animals ranging from cryptic, benign infections to spectacular, massive infections that cause extensive damage and often death of the host. Microsporidai are opportunistic pathogens in patients with AIDS most commontly causing diarrhea, encephalitis, myositis, or conjunctivitis. Microsporidia can also cause infections in other immune compromised hosts, such as patients who have undergone organ transplantation or those on immune modulating therapies. They are also capable of infecting immune competent hosts most commonly causing keratoconjunctivitis or diarrhea. These pathogenic organisms are classified as NIH category B priority pathogens and EPA pathogens of interest as they are transmitted by both food and water sources. In addition to being human pathogens, microsporidiosis has major economic impacts on agriculture (via effects on insects and sericulture), aquaculture and animals (food, domestic and wildlife). Microsporidia produce spores with a unique invasion mechanism, the polar tube, that is one of the most complex single celled forms known in the biological world. The mechanism by which the polar tube interacts with the host cell during invasion is still unknown. A long standing research program in my laboratory group is focused on understanding the mechanism of invasion and the structural biology and composition of the polar tube. We have developed techniques for the purificaiton of this structure, identified polar tube proteins (PTPs) and their post translational modifications and how these proteins interact. Futhermore, our studies have begun to define the functional roles of these proteins in the structural biology of the polar tube and the process of invasion. However, the full complement of proteins in this structure and the interactions of these components during invasion remain to be determined, In other microbes studies on invasion have provided key data for understanding pathogenesis and for new therapeutic approaches to the management of infections. We have demonstrated that the major polar tube protein is O-manosylated, a post translational modification that is involved in invasion, that inhibiting binding of manose can limit infection, and that antibody to polar tube protein 1 (PTP1) can block invasion demonstrate that targeting the invasion organelle is a way to limit infection. The proposed research project will employ a combination of proteomic, immunologic and ultrastructural studies to characterize the polar tube and its protein interactome to better define and study the mechanism of invasion. We will also evaluate the ability of a newly identified polar tube protein (PTP4) to bind to host cell components and use antibody to PTP4 as a marker to identify the area of the cell at which invasion is occurring. This will facilitate a detailed correlated microscopic analysis of the mechanism of invasion by these pathogens. Furthermore, electron microscopic techniques will be employed to provide insight into the three dimenstional structure of the proteins making up the polar tube providng critical information on fundamental questions concerning the organization of this invasion organelle that have not been able to be resolved by traditional microscopy. Studies of the composition, formation and function of this organelle during germination and invasion should provide a basis for the development of new strategies for control of these important parasitic protists.